1. Field of the Invention
The present invention relates to a method and an apparatus for supplying a predetermined amount of mercury to a chamber part of an arc tube during production of the arc tube which is used for a discharge lamp as a light source of a lighting tool such as a head lamp for an automobile, the arc tube having a sealed chamber part with electrodes provided facing each other, a light emitting substance such as mercury and a metal halide sealed therein.
2. Description of the Related Art
FIG. 10 is a cross-sectional view of a conventional discharge valve to be used in a head lamp for an automobile comprising an arc tube for a discharge lamp. An arc tube 1 comprises a sealed chamber part 2c including electrode bars 3a, 3b facing each other, a light emitting substance such as mercury and a metal halide sealed therein, formed between a pair of front and rear pinch seal parts 2a, 2b.
An insulating base 5, and a pair of long and short metal lead supports 6a, 6b for supporting the front and rear end part of the arc tube 1, with lead lines 4a, 4b, elongating from the arc tube 1 electrically connected to lead supports 6a, 6b. A globe G surrounds the arc tube 1 and blocks ultraviolet rays that are emitted from the arc tube 1 that can be hazardous to the human body. The mark 2e is the trace of the pinch part remained on the chamber part 2c at the time of pinching the straight tube provided for introducing mercury or a metal halide.
Since the amount of the mercury sealed in the sealed chamber 2c influences the value of the lamp voltage, which is one of the characteristics of the arc tube, it is preferable that a predetermined amount of mercury is supplied into the chamber part 2c in the production process of the arc tube in order to obtain a rated lamp voltage.
As shown in FIGS. 11A and 11B, a conventional method for supplying a predetermined amount of mercury to a chamber part of an arc tube comprises the steps of inserting a tube 7 with a mercury droplet P, held at the tip part by adsorption, through a straight tube 2d integrally bonded with a chamber part 2c of an arc tube, to the vicinity of the chamber part 2c, and poking the mercury droplet P with a poking bar 8 fitted in the tube 7 for dropping the same into the chamber part 2c.
For holding the mercury droplet P at the tip part of the tube 7 by adsorption, as shown in FIG. 12A, the tip part of the tube 7 is put in the mercury liquid surface in a container 9 filled with mercury so that a predetermined amount of mercury is introduced into the tube tip part by withdrawing the poking bar 8. Then, by lifting up the tube 7 above the liquid surface, as shown in FIG. 12B, a mercury droplet P can be held by adsorption at the tube tip part, utilizing the surface tension and the vacuum.
However, according to the conventional method and apparatus for supplying mercury, since the operation for holding the mercury by adsorption needs to be executed at a site different from that for ejecting the mercury, a problem is involved in that the operation is troublesome as well as time-consuming.
Furthermore, since it is necessary to put the tip part of the tube 7 into the mercury liquid surface in the container 9, withdraw the poking bar 8 for introducing the mercury into the tube 7 tip part, and lift up the tube 7 and move it above the straight tube 2d, the apparatus structure is complicated.
Moreover, according to the conventional method and apparatus, since the mercury is measured by withdrawing the poking bar 8 with respect to the tube 7 for introducing a predetermined amount of mercury into the tube 7 tip part and it is supplied into the chamber part 2c by poking the mercury droplet P by advancing the poking bar 8 with respect to the tube 7, the mercury cannot be supplied in accurate amounts. That is, the bulging shape of the lower part of the mercury droplet held by adsorption at the tube tip part differs each time, and thus the amount of the mercury droplet held by adsorption is often irregular. Furthermore, in some cases, the mercury may adhere and remain on the tube 7 tip part to the poking bar 8 tip at the time of dropping and supplying the mercury droplet P into the chamber part 2c so that all the mercury droplet held by adsorption may not be supplied into the chamber part 2c.
Furthermore, the volume of the chamber part 2c is as small as about 20 to 50 .mu.l in an arc tube for a discharge valve to be used in a head lamp for an automobile, and the mercury amount to be sealed is slight (1 mg or less). Therefore, the tube 7 and the poking bar 8 need to be processed extremely narrowly. This process is difficult. Furthermore, the technique for smoothly sliding the narrow poking bar 8 in the narrow tube 7 is also demanding.
Accordingly, as a result of the problems of the conventional technique, the present invention supplies a predetermined amount of mercury, corresponding to the high pressure gas application time, by filling the inside of a small diameter pipe with mercury and pressing and ejecting the mercury in the small diameter pipe with a high pressure gas.